Constant output current control is an object that power supplies would like to achieve. For example, for power supplies that provide driving current for illumination with constant brightness, this driving current should be a constant, substantially unchanged if the maximum voltage of the line voltage supplied to the power supplies varies from 100 VAC to 200 VAC.
Regarding to contemporary power supplies, power factor is also an issue that designers should concern. Simply speaking, a power supply with an excellent power factor acts in a way like a linear resistor, which, if supplied with a line voltage, conducts a line current in phase with the line voltage. A power supply with a good power factor provides power factor correction (PFC) to efficiently utilize the electric energy that could be conveyed from an electric power plant.
It is up to the innovation and skill of circuit designers to achieve both PFC and constant output current control.
FIG. 1 illustrates a switching mode power supply (SMPS) 8 in the art, whose topology is a buck converter. In FIG. 1, the combination of a capacitor and a light-emitting-diode (LED) chain with LEDs exemplifies output load 16.
Bridge rectifier 12 rectifies alternating current (AC) voltage source from lines AC into direct current (DC) voltage, outputted at line voltage power node IN. Due to the sinusoidal waveform residing across lines AC, the waveform of line voltage VIN at line voltage power node IN has an M-like shape. Inside converting module 10, primary winding PRM energizes if power switch 15 performs a short circuit; primary winding PRM de-energizes through diode 11 if power switch 15 performs an open circuit. Feedback module 20 detects the driving voltage across output load 16 to, via photo coupler 23, control feedback signal VFB at node FB. Controller 18 could be a pulse width modulator (PWM) and control the current flowing through primary winding PRM according to feedback signal VFB. Operational power supply 14 with auxiliary winding AUX generates operational voltage VCC that powers controller 18. Controller 18 detects current sense resistor 24 to provide gate signal VGATE, determining whether power switch 15 is an open or short circuit.
FIG. 2 exemplifies controller 18 in the art. Logic control 82 generates gate signal VGATE according to the results from comparator 88 and clock generator 87. As known by persons skilled in the art, feedback signal VFB is a kind of limiting signal because it substantially controls or determines the peak voltage of current sense signal VCS (at node CS) and the peak current flowing through primary winding PRM.